I built this sculpted prim arch using a Loft in Curvy. Read how below:

1) Select the Loft tool
2) Draw the large curve for the opening under the arch in Front view (I only made 1/2 an arch at a time)
3) Draw a C-shape curve at the base of the arch. You can draw this in the top view and move it into position.
4) Use the ruler tool to add straight edges, and the widget move tool with a small size to tweak the shape of the curve.
5) If you want a solid arch, move the endpoints so they line up
6) Select the C-shape curve and choose Curve/Clone from the menu.
7) Move the cloned curve into place at the top of the arch, rotating it round to vertical.
8) If your arch looks inside out when you turn off Double-Sided in object properties, you can turn the C-Shape curves the other way around.
9) [BETA] Set the resolution high and use Ambient Occlusion to automatically make shade maps and AO texture.
10) Export as sculptie for Second Life (Or use in your Curvy art!)